This invention relates to digital voice/data/control switching systems and more particularly to circuitry for communicating voice data and control information to and from a local communication station in a serial, time multiplexed signal stream.
As the complexity of data communications networks increases, the need for simpler and more economical methods of interfacing the various devices on the networks becomes critical. Simple interconnection between two devices such as two telephones or such as a computer and a terminal can be accomplished quite easily. For example, communications can be established between devices with one wire, such as was done with an early telegraph system. Though each device had simultaneous access to the one wire, typically connecting a remote junction box to a central switching location, only one device at a time could use the wire to send messages to other devices. When the device relinquished the line, another device could then use the line to send messages. Since only one device could use the line to send at any one time, the number of messages which could be sent would be quite low. Thus, the data rate of the systems was low. Some protocol was established for determining which device had the use of the line at any time. Considering the relative simplicity of such a system and the low data rates, and the fact that the system was most likely under the jurisdiction of one entity, such as the telegraph company, the protocol could be as simple as listening to determine whether the line was in use. As the volume of signal traffic on the line bacame greater and the need for immediate availability of communication paths increased, alternative systems came into use.
Most present day switching systems employ star architectures or distributed star architectures. In the star architecture, a large central switch is employed and all stations are wired to a central location. At the central location, a communications line from one device could be connected to the communications line from another device to which communication was to be established. This could be done manually as with an operator at switchboard, electromechanically such as in a complex telephone crossbar system, or under computer control as is done in modern telephone networks.
Although the method of running communications lines from each user to a central location has many advantages, it has the distinct disadvantage of requiring a communications line from each device to a central location. Thus, although two devices might be relatively close to each other compared to the distance to the central location, communications between the two devices would be routed through the central location. In a large, widespread network this would require substantial expenditures for the communications lines that would remain idle much of the time. If another device has to be added to an existing system a new dedicated communications line would have to be added to connect the device to the central location. Moreover, since the entire system was dependent upon proper operation of a central switch, the survivability of the system was low. It is obvious that such a method of interconnecting communications devices has substantial economical and practical drawbacks.
A distributed star still employs a large central swtich but reduces the wiring requirements to that switch by multiplexing many conversations, or circuit paths, onto the wiring between the central switch and peripheral switching units. The advantages of distributed switching include improved reliability, improved availability, improved survivability, and reduced installation in wiring costs. However, a fundamental obstacle has prevented the widespread adoption of distributed switching, that obstacle being connectivity or allocation of circuit paths among the various nodes of the network. The connectivity between peripheral switching units remains limited to the number of circuit paths or "party lines" carried on the multiplexed wiring between the central switch and the peripheral switching units (PSU). The users in a given area would have to wait until the party line to the central switching location was not in use before initiating a message. As was frequently the case, a large number of users could want to send more messages than such a system could handle in a given time frame. Connectivity is not a problem when the switch is non-blocked, i.e. there is a circuit path for every station in the system, such as in the earlier star architecture. However, the cost of a non-blocked system is excessive and some level of blocking is introduced in order to reduce the cost per station of the system. The challenge is therefore to provide a communications system that employs some degree of blocking to avoid costly redundancies, but does so in a widely distributed manner to avoid bottlenecks in the signal traffic.
In the distributed star, blocking may be introduced in the PSU. Consequently, all stations on the PSU may contend for the number of circuits that exist between the PSU and the central switch. Typically the number of calls initiated per unit time varies from one PSU to another. To obtain a desired grade of service, i.e. the likelihood that a circuit path will be available for a call, it is thus necessary to balance the load on the PSU's by physically changing the number of telephones wired thereto. Because of the dynamic nature of modern businesses, the offered load to each PSU changes with time necessitating an ongoing process of traffic analysis followed by physically disconnecting and reconnecting telephones from one PSU to another. That process is costly, time consuming and introduces reliability problems.
When the number of circuits is small a significantly larger ratio of circuits to telephone sets (telesets) is required to ensure a fixed grade of service. For example, a 30% grade of service is insured by providing three circuits to service ten telsets, but requires two circuits to service four telsets. Therefore, in order to minimize the number of dedicated circuits, a more desirable system would allow the number of circuits per PSU to vary dynamically in accordance with the number of associated telesets. Ideally, the system would allow for the offered load from a local station to contend for all the circuits of the central switch rather than to contend for the small and fixed number of circuits at an individual PCU.
One approach to the problem developed in the prior art is the use of time division multiplexing of digital data. In a system which uses time division multiplexing a communications line is not provided from each device to a central location. Instead, each device is connected to other devices relatively close to it. Thus, there are considerable savings in the number of communications lines needed to interconnect the devices in a network. All the devices in a communications network may be connected in a ring, chain or the like, with each device connected to two other devices, or to one other device in the case of a device at the end of a chain connection. Although it might appear that such a device would then only be able to communicate with a device to which it has a direct connection, each device can communicate with all other devices connected to the network. The network ring or chain is continuous, with each device either tapping the ring or chain, or forming a part of the ring or chain. Although the devices are physically connected to the same line at any one time, and do not transmit data at the same time, the devices may be time multiplexed and need not have to wait until other devices complete their messages before sending their own messages. Communication between devices can typically be accommodated using only a portion of the time available in each cyclical message frame, thereby allowing the ring to communicate numerous messages within a given period. Moreover, the message pool includes all the resources of the network signal stream. Thus, blocking is provided on the most distributed basis possible.
When time division multiplexing is employed the communications line is not assigned solely to one device until the completion of a message. Instead, the line is assigned to each device for a relatively short period of time, typically referred to as a time slot. Other devices in the communications network are likewise assigned to time slots. The time slots occur periodically on the communications line, and are repeated at a frequency such that the device can send or receive data continuously at its normal data rate. A message frame is comprised of all the time slots available for devices.
In an exemplary system utilizing time division multiplexing, the communications devices might operate at a data rate of 1000 bits per second (bps). A communication line operating at 100,000 bps would be able to transfer messages to or from this device and 99 similar devices in a message frame which has a 1000 hertz repetition rate. The data from each device would be assigned to each of the 100 one-bit time slots in the message frame. Other configurations could assign the time slots for devices in multiple-bit groups.
In either the contemporary centralized network or the contemporary ring network, however, devices can only be added to the system if there are time slots available in a message frame. Therefore, it would be difficult, if not impossible, to add the 101st device to the exemplary system if the available time slots are permanently assigned to other devices. In many communications applications the devices present in the system probably would not all be communicating at the same time. Thus, there could be a substantial number of the time slots idle at any given time. However, a typical prior art communications system would not have the flexibility to reassign the idle time slots to additional devices to take greater advantage of the available time slots. Although increasing the number of time slots would accommodate extra devices, if possible to do so, the incremental increase in the number of time slots might be large compared to the number of devices to be accommodated, and therefore could result in a large number of unused time slots.
Another problem with the typical prior art system utilizing time division multiplexing is that devices operating at different data rates cannot be accommodated. Although the time slot allocations in a message frame might be adequate for most devices in a system, there is often a need for other devices operating at higher or lower data rates. For example, a system might consist primarily of digitized telephones operating at 64,000 bps. If a typical network is configured to accommodate the telephones, it might not be able to accommodate the communications to and from a terminal device operating at other data rates, e.g. 19,200 bps. Furthermore, a device might be such that it operates at different rates when communicating with different devices. Thus, a time slot assignment sufficient to accommodate a 19,200 bps data rate would be partially unused if the device were to operate at 9600 bps or a lower data rate. Similarly, a time slot assigned to a terminal device operating at 9600 bps would not be able to accommodate the same device operating at 19,200 bps.
A voice signal can be transferred without any apreciable loss of quality as a stream of 64,000 data bits per second (64,000 bps). The voice signal is sampled at periodic intervals by the sending device; the samples are converted to a digital format; the digital data is transferred to the receiving device as a stream of data bits; and the digital data is converted to a voice signal by the receiving device.
In comparison to a voice signal, the transmission of character information between a computer and a high-speed video terminal can require data transmission rates in the range of 19,200 bps. On the other hand, a typical teletypewriter terminal might only require data at a rate of 110 to 300 bps to operate at full capacity.
Typically a data communications network, therefore, needs to be capable of handling data rates from 110 bps to 19,200 bps, and under some circumstances up to 1,000,000 bps or more.
As can be readily seen from the foregoing, the implementation of time division multiplexing in the prior art accomplished a significant savings in physical resources in a typical communications network. However, the prior art systems have serious limitations with regard to flexibility in light of the ever increasing demands on data communications systems, both with regard to the increases in the quantity of devices to be connected to a system and with regard to widespread variations in the communications rates used by those devices.
In practice, a user at a given location will have both voice and data communications equipment which may be alternately, or simultaneously used. Preferably, the communications equipment connecting the local station to the network should be able to accept either or both voice and data information, format information for communication to the network, and synchronize the data rates of that information with the network data rate. The equipment should also have the ability to dynamically modify the network bit space allocation assignable to the particular communications device in accordance with the operating requirements of that device. The equipment should preferably be able to effect those functions without the need for extensive control equipment at the local equipment, and without the need for connecting the local equipment to dedicated control lines. The system should have the ability to communicate control information to and from the network controller using the same lines used to communicate voice and data information, thus simplifying the connection requirements for individual communications devices.
In contemporary communications systems interconnection of devices that operate with different communications formats and information rates is accomplished through the use of interface devices that perform a specialized function and operate with only one or, at most, a small number of terminal devices. Generally, such interface devices are hardwired with regard to formats and rates, or are manually switchable. Such devices do not lend themselves to control by a central network controller and do not provide the requisite flexibility in the rapidly expanding communications field.